1. Field of the Invention
The present invention relates to a digital broadcasting receiving unit and a digital broadcasting system.
2. Description of the Background Art
Conventionally, a digital broadcasting receiving unit (hereinafter referred to as receiving unit) for home use has been so constructed that a base station and the receiving unit are operated according to a common clock and time information by accurately synchronizing the reference clock of the base station (broadcasting station) with the reference clock of the receiving unit at home. This method has been described in MPEG2 standard (ISO_IEC—13818-1, Appendix D).
This receiving unit utilizes an expensive crystal oscillator having a variable frequency (hereinafter referred to as VCXO) in order to accurately synchronize the reference clock of the base station with the reference clock of the receiving unit. The operation of the receiving unit having the VCXO will be briefly described as follows.
First, a difference between time stamp information (PCR: program clock reference or SCR: system clock reference) sent from the base station and time information (STC: system time clock) counted by the reference clock (for example, 27 MHz, which is generated by the VCXO) of the receiving unit side is detected. Then, the frequency of the VCXO is controlled using that difference. Here, the receiving unit constitutes a feedback system for time information error and a phase synchronous loop is realized.
The frequency of broadcasting the time stamp information from the base station is very low or may be irregular. The time stamp information may be lost under bad receiving condition. For example, the frequency of broadcasting the time stamp information is about once per 100 ms. Assuming that the frequency of the reference clock of the receiving unit side is 27 MHz, the time stamp information is broadcast only once every about 2,700,000 clock cycles and the reference clock of the receiving unit side needs to be synchronized with the reference clock of the base station according to difference information at this time. When the time stamp information is lost, the frequency of detection of the difference information is lowered further.
On the other hand, because color shift of an image, noise, noise of sound or the like is generated easily when the frequency deflection of the receiving unit side clock is large, the frequency deflection of the receiving unit side clock needs to be kept as small as possible. For the reason above, the frequency needs to be controllable in order to generate the reference clock on the receiving unit side and a clock oscillation circuit having a high frequency accuracy and a high stability is necessary. Conventionally, the VCXO is used in such a clock oscillation circuit.
In the meantime, the reference clock of the base station and the reference clock (generated by the VCXO as described above) on the receiving unit side after synchronization is, for example, 27 MHz.
As described above, in the conventional receiving unit, the expensive VCXO is an indispensable component, which causes a rise of the price of the receiving unit.
For the reason of physical composition, the VCXO cannot be incorporated in a decoder LSI and such an external component as a low-pass filter is also indispensable. Consequently, this not only causes a rise of part cost but also induces a trouble.